Theoretical modeling of the Al paramagnetic center and its precursors in stishovite |
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| Authors: | Sanda M Botis Yuanming Pan |
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| Institution: | (1) Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, S7K 5E2, Canada |
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| Abstract: | Previous electron paramagnetic resonance (EPR) spectroscopic study of gamma-ray-irradiated stishovite at 77 K detected an
Al hole center, which was proposed to be an O2
3−–Al3+] defect. First-principles quantum-mechanical calculations show that the unpaired spin is 85% localized on one of the six
oxygen atoms at an AlO6 octahedron, while the calculated 27Al hyperfine constants are similar to those determined by EPR experiments. Theoretical results allow us to propose the Al
center to represent an AlO6]0 defect, and hole hoping among equivalent oxygen atoms is responsible for its detection only at cryogenic temperatures. Theoretical
calculations also show that the diamagnetic precursors AlO6/H+]0, AlO6/Li+]0, and AlO6/Na+]0 are stable in stishovite. The calculated OH bond distance and orientation are in excellent agreement with those inferred
from FTIR spectra and previous theoretical calculations. The calculated AlO6/Li+]0 and AlO6/Na+]0 defects suggest that the monovalent cations such as Li+ and Na+ are potentially important in accommodating Al in stishovite in the lower mantle. |
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